Imidazolines are a family of compounds based on a five-membered ring structure containing two nitrogen atoms and a double bond. The ring is numbered in such fashion that the nitrogens carry the lowest combinations of numbers: ##STR2##
Commercially, imidazolines are made from the reaction of fatty acid, fatty alkyl (e.g. methyl) esters, or fatty triglycerides with a polyamine such as, diethylenetriamine (DETA), aminoethylethanolamine (AEEA), ethylenediamine (EDA), or triethylenetetramine (TETA). The intermediate amidoamine is dehydrated to yield the cyclic imidazoline product.
While the manufacture of imidazolines on a commercial scale is relatively easy, the product is not easy to store or use without hydrolysis. Numerous authors have commented on the instability of the imidazoline molecule. Linfield, JAOCS, 61, No. 2 (1984), p 439, states that the imidazolines are unstable and in the presence of water revert to the amidoamine starting material by standing overnight at room temperature. Wechsler, et al., U.S. Pat. Nos. 4,269,730 and 4,189,593, caution that during the reduced pressure dehydration to make the imidazoline, care must be taken to avoid any contact between the reactant and air which would cause rapid and severe darkening of the product. Butler, et al., J. Chem. Res., (5), 84 (1981) report decomposition of the imidazoline ring under atmospheric conditions in 2-9 days provided the compound contains a cis-olefin system. Bristline, et al., JAOCS, 60, No. 4 (1983), p 823, showed that the imidazoline ring content in a system decreased from 38% to 6% imidazoline in 72 hours with the addition of 2% H.sub.2 O (half-life of 24 hours). Even in a sealed container, these authors reported 5-8% loss in ring content over 18 months. Their conclusion was that, "When used as intermediates, imidazolines should be reacted promptly and prolonged storage should be avoided". Bristline, et al., JAOCS, 60, (1983), p 1676, showed that, "The imidazoline is hydrolyzed quantitatively to the diamide in the presence of water in ca. four days at room temperature."
This well-documented hydrolytic instability has inhibited the commercialization of imidazolines for aqueous applications. When imidazolines are protonated or quaternized, however, their hydrolytic stability is dramatically increased as is their water compatibility. Commercial producers, then, often manufacture the protonated or quaternized derivatives of imidazolines. Imidazolone-derived amphoterics are known to be excellent foamers and good cleaners, yet are substances of low toxicity possessing properties of low-irritancy to both skin and eye. Hunting, "Amphoteric Surfactants", Cosmetics & Toiletries, 95, November, 1980, p 95, and references cited therein, reports that these amphoterics also are bacteriostatic.
Certain fatty imidazoline derivatives are liquid material at ambient temperature. These materials are derived from highly unsaturated fatty acid sources such as, for example, tall oil, soya oil, and oleic acid. These types of imidazolines usually are used in applications where light color is not required as they tend toward darker colors due to the characteristics of the sources for unsaturated fatty acids and oils. Hydrogenated fatty acids and esters typically yield light colored products, but also produce higher melting point materials when converted to imidazolines.
Despite the foregoing, commonly-assigned application U.S. Ser. No. 07/155,768, filed Feb. 16, 1988, discloses an imidazoline product which is light-colored and color-stabilized. Such product is formed by pre-treating the polyamine reactant with an effective amount of a hydride prior to the amide-forming reaction with a fatty acid or ester. The resulting stabilized imidazoline product is reported to be storable as a liquid under conventional liquid imidazoline-storage conditions for a sufficient time to permit the product to be shipped from the manufacturing site to another location whereat the imidazoline product is to be used. This time period is reported to range from a few days to two weeks or so. Despite this significant advance in the handling and storage of liquid imidazolines, further extension of shelf life is desirable.